Devices for detecting the presence of objects have been available for several years and have been used in various applications. Security systems for both home and commercial settings use various sensors for detecting intruders. In the manufacturing industry, object sensors have been used for various purposes, e.g., detecting objects along an assembly line. Typically, manufacturing sensing systems are set up to detect the presence or absence of an object in a certain selected region. One specific manufacturing application involves the assembly of cardboard boxes. Assembly lines exist for automatically forming cardboard boxes, i.e., forming a box with a bottom and four sides. However, because such assembly lines are not fail proof, defective boxes are occasionally produced. Some assembly lines incorporate a sensor system for automatically detecting defective boxes. The detection systems detect whether each of the sides of a box has been properly erected. Such sensing systems incorporate several sensors for detecting the presence or absence of box sides.
One category of object detection devices currently available are photoelectric transceivers that emit and sense light. Such a transceiver is oriented to emit light into a preselected region. If there is an object in the region, the emitted light is reflected off the object. When the transceiver senses that the light it emitted has been reflected back, the transceiver provides an output signal indicating that an object is present.
One type of photoelectric transceiver emits pulses of light, as opposed to continuous light, so that constant ambient light does not affect the transceiver. The sensing of light by such transceivers is synchronized with the emission of light pulses as only light pulses sensed at the times that light pulses are emitted are indicative of an object. While such sensors are not affected by constant ambient light, other types of noise can cause the sensors to give false detections. Interfering noise can come from several sources, including other pulsating light sources, external electromagnetic noise, and electrical noise within the sensing device itself. In many applications, including manufacturing, false indications of an object's presence can cause serious problems. In such applications, it is important that the detection system be immune to noise.
While photoelectric transceivers currently available have some noise immunity features, they are not completely effective, particularly in the presence of synchronous or nearly synchronous noise. Other transceivers operating nearby are one example of a source of synchronous noise. In most object detection systems, several transceivers are used. For example, in the previously described application of cardboard box manufacturing, at least four transceivers are used to detect each of the four box sides. Unfortunately, one of the transceivers may detect the light emitted by one of the other transceivers resulting in transceiver interference. Transceiver interference can cause a transceiver to falsely indicate that an object is present. The present invention provides a low-power photoelectric transceiver that solves this and other noise problems. The transceiver of this invention is highly insensitive to both synchronous and asynchronous noise.
Additionally, photoelectric transceivers are adjusted during both initial installation and periodic maintenance of the detection system. The gain of a transceiver is adjusted so that the transceiver detects objects only in a desired region and not, for example, some background object such as a wall. A technician adjusting the gain of a transceiver needs some sort of indication of the strength of back-reflected light that a transceiver is receiving. Transceivers are currently available with signal strength indicators that serve this function. The very first and simplest photoelectric signal strength indicator was simply an on/off indicator which told you that you either had enough or did not have enough signal strength for detection. This signal strength indication was one and the same as the detection indication, i.e., the sensor output status indicator. Numerous improvements have since been offered in the prior art for a variety of purposes. One such method is the subject of the Fayfield U.S. Pat. No. 4,356,393 which teaches an LED indicator that blinks at a rate proportional to the signal strength. Another method is the subject of the Warner U.S. Pat. No. 4,644,341, which teaches the addition of an LED bar graph display to indicate the minimum and maximum light received and thereby the contrast. Another method is the subject of the Juds U.S. Pat. No. 4,851,660, which teaches an LED indicator which is driven to produce a brightness proportional to the logarithm of the detected signal strength. A stability indicator is one type of signal strength indicator. One stability indicator found in the prior art uses comparators to turn on an LED stability indicator when the received signal strength is in a window defined around the detection threshold.
Unfortunately the prior art of the Fayfield U.S. Pat. No. 4,356,393 and the Juds U.S. Pat. No. 4,851,660 patent, while being useful during the initial installation and alignment of the sensor, are not useful for preventative maintenance of ongoing operations because the relative LED flashrate (Fayfield) and the brightness (Juds) as judged at a later time is so subjective that one cannot tell by observing the LED indicator how much margin remains before slight vibration or dust build up will result in detection failure. The LED bar graph of the Warner U.S. Pat. No. 4,644,341 will indicate marginal operation prior to detection failure, however, the combination of required space for the bar graph display and the attendant cost of the display render it impractical and non-competitive for use in modern miniature low cost photoelectric sensors. Prior art stability indicators which do address the aforementioned problems implement fixed analog comparator thresholds, typically around twice and half of the detection threshold in order to generate the stability LED indicator signal. This method, unfortunately, makes an a priori assumption about both the optical cleanliness of the sensor environment and the electrical and optical noise present in the sensor environment. By fixing these thresholds, any of numerous low contrast applications in clean benign environments will indicate unstable operation even though this is not the case.
The present invention overcomes the aforementioned problems through a digital implementation which indicates unstable operation. The digital stability indicator provides correct stability indication even in low signal-to-noise contrast situations. Furthermore, implementation by digital means as described herein provides for a smaller less costly circuit.